1. Field of the Invention
The present invention relates to a passivation structure of a display device, and more particularly, to a passivation structure formed of an organic/inorganic film.
2. Description of the Prior Art
The progress of science and technology has led to organic materials being well applied to all kinds of electrical devices. For example, organic light-emitting displays (OLEDs), which are formed by using organic materials, have the advantages of simpler structures, excellent operating temperature, high contrast, and a wide viewing angle, and have the beneficial characteristics of light-emitting diodes (LEDs), such as rectification and luminosity, so as to be used extensively in the field of display devices. Since the OLED uses luminous devices formed of organic materials to provide a light source, the OLED is very sensitive to the moisture. Once the organic light-emitting devices are exposed in the moisture, the cathode thereon may be oxidized and the interface of organic compounds may be peeled. This leads to dark spots being generated in the luminous devices, which deteriorates the brightness and the lifetime of the display devices. As a result, the package material used to package the electrical devices not only needs high anti-abrasiveness and thermal conductivity, but also requires low moisture permeability to prevent the organic materials from being exposed in the external environment effectively and to improve the lifetime of the electrical devices.
For example, in the conventional package process of display devices, a sealing agent made of polymer materials is often used to combine the container, which is composed of a metal or glass material, with the substrate and a desiccant agent and dry nitrogen are filled into the empty region there between. However, this package structure can be only applied to the display devices with metal or glass substrates, but cannot be used in packaging those with the flexible substrates. In addition, the metal container has disadvantages of having a heavy weight, and being oxidized easily. In the fabricating process, the metal container also has disadvantages of pealing off from the glass materials easily and having the requirement of a high degree of flatness. The glass container has the disadvantages of having heavy weight, cracking easily, and pealing off easily due to stress differences. Moreover, most of the sealing agents composed of polymer materials lack adequate protection from moisture. As a result, although the electrical devices are packaged, the moisture of the external environment still permeates into the packaged device gradually and erodes the display devices so as to deteriorate the display effect and decrease the lifetime of the display devices.
In order to solve the above-mentioned problems of the metal or glass container, a new passivation process that utilizes films to encapsulate the protected devices is developed. Please refer to FIG. 1, which is a cross-sectional diagram of a passivation structure 16 disclosed in U.S. Pat. No. 5,811,177. As shown in FIG. 1, an OLED 10 mainly comprises a substrate 12, a display unit 14 positioned on the substrate 12, and a passivation structure 16 covering the display unit 14 and the substrate 12. The display unit 14 is composed of a plurality of pixels and further comprises a driving circuit (not shown) disposed on the substrate 12 for driving the pixels to display. The passivation structure 16, which is a multiple film structure, comprises a metal layer 18, a buffer layer 20, a thermal coefficient matching layer 22, a low permeability layer 24, and a sealing layer 26 stacked on the display unit 14 in sequence for protecting the display unit 14.
Furthermore, another passivation structure which utilizes a metal layer, inorganic materials and hydrophobic polymer materials is disclosed in U.S. Pat. No. 5,952,778. Another moisture-proof multi-layer structure is disclosed in Chinese Taipei Patent 379,531 to improve the above-mentioned problem. The structure includes a moisture-adsorbing resin layer, an adhesive layer, and a transparent resin layer and covers an electroluminescent device to prevent the electroluminescent device from moistening and oxidizing.
As mentioned above, although most inorganic materials have a superior water repelling ability, the inorganic materials have a significant difference from the organic light emitting display unit, which is almost formed of organic materials, in the stress or the thermal expansion coefficient. The adhesion between the inorganic materials and the organic materials is also weak so that the inorganic materials are easily peeled from the organic light emitting display unit. As a result, though the conventional passivation structures have different package design or package materials, they typically utilize a polymer material as a buffer layer and stack the buffer layers and the inorganic materials in a staggered layout on the display unit to form an multi-layer passivation structure to prevent electrode materials or organic materials in the display device from being eroded or oxidized by the moisture and oxygen in the external environment. Normally, some moisture sensitive electrical devices, such as the OLED, require a passivation whose permeability is less than 0.05 g/m2 day. Thus, most of the conventional passivation structures are composed of more than five stacked layers to meet the permeability requirement. However, although the multilayer structure can provide a better effect on moisture protection, there is the disadvantage of complicated fabricating process which leads to a high fabrication cost and long fabricating time.
Additionally, since the passivation structure 16 on the display unit is usually opaque, the organic light emitting display device has to use the transparent substrate beneath to display images in a bottom emission mode. When the size of the display device increases and the resolution thereof improves, the display device operates in an active driving method instead of the conventional passive driving method. In an active organic light emitting display device, each pixel needs an independent driving circuit so that more electrical devices are required and more area is occupied thereby. This leads to a decrease in the ratio of transmitting area in each pixel. As a result, when light beams generated from the organic light emitting display device pass downward through the transparent substrate to display images, some light beams are blocked by the driving circuit in each pixel so that the brightness of the organic light emitting display device is reduced and the display performance is therefore deteriorated. Thus, it is important to develop a new passivation structure and method thereof to solve the aforementioned problem.